Post navigation

Easy Mac, Spaghettios, and left overs…What would we do without microwave ovens?

I am a senior biochemistry student at DePauw University in beautiful, small, middle of no-where Greencastle, IN. I am originally from Knoxville, Tennessee, so if you’d like you can imagine this post was written in a Southurn accint. My background is mostly in biochemistry, but as an aspiring med student I’ve also taken two semesters of physics. I did pretty well in those two semesters, and I’m confident I learned enough to research the basics of microwave physics and explain them to you here. I have also have an extensive history with microwavable food…I currently eat Lean Cusines and Campbell’s Soup on a regular basis while I’m at school…so I’d like to think you can trust me when I say I am a microwave expert of sorts.

Everyday when my brother and I came home from school he would open a can of Spaghettios, pop them in the microwave, and wait the 2 or 3 minutes until they were warm and ready to eat. When I was younger I just inherently knew that if you put something in the microwave and turned it on for long enough the food would get warm, but how does a microwave oven actually heat up your food?

Microwave ovens actually use microwaves- a form of electromagnetic radio wave to heat food. Electromagnetic radiation is a form of energy that has both electric and magnetic field components and exhibits wave-like behavior as it travels through space (4). Radio waves have the longest wavelength and the lowest frequency in the electromagnetic spectrum (seen below), making them the lowest energy EM wave based on the Plank-Einstein relationship (E = hc/λ) which says the energy of an electromagnetic wave is directly proportional to the Planck Constant (h) and the speed of light (c) but indirectly proportional to wavelength (4).

In the case of microwave ovens, the frequency of radio wave usually used is about 2,500 megahertz (4). Interestingly, microwaves at this frequency are absorbed by water, fats and sugars and are not absorbed by most plastics, glass or ceramics. As the water, fats, and sugars in your Spaghetios or Pop-Tart absorb the microwaves they heat up by a process called “dielectric heating.” The molecules are dipoles, meaning they have a positive and negative charge on opposite ends. The dipoles begin to spin as they try to align themselves with the alternating electric field of the microwaves, causing them to rub together and create heat (3). The heat produced by the the water, fat, and sugar molecules in your food rubbing together begins to heat the molecules around them and, essentially, to cook your food! The length of time required to cook your food, therefore, depends on its water, fat, and sugar content in relation to its size.

…So, where do the microwaves come from? The microwaves are generated by a magnetron within the oven. Magnetrons were invented in 1921 and then vastly improved in the 1940s (2). The physics of a magnetron is a little beyond the grasp of this blog, but in layman’s terms it is essentially a tube that moves electrons through a magnetic field which causes the electron path to curve and create oscillating microwaves (2).

The microwaves are then corralled into the cooking box by the waveguide where they bounce around, reflected by the metal of the box, until they are absorbed by your food. Microwaves are a common household object around the world. We use them to heat up left-overs, cook microwave dinners, and to pop popcorn before we sit down to watch a movie. I hope you enjoyed learning about a little bit about the science behind these household marvels, but in case you’re craving a little bit more knowledge, here are some fun facts…

Microwaves cook your food from the inside out, as opposed to a conventional oven which cooks food from the outside in by the process of convection. This is why Hot Pockets have a little metal casing around them which allows heat to be reflected back at its surface creating a crust! (1)

Microwaves aren’t nearly as efficient at cooking frozen foods because the molecules are not free to rotate. (3)

As of 1971 only about 1% of American homes had a microwave. That number rose to 25% by 1986, and as of 2009 90% of American households had a microwave.

Microwaves convert Vitamin B12, an essential vitamin predominantly found in meat, to an inactive form. (3)

However, spinach retains almost all of its folate when cooked in a microwave, but loses approximately 80% when it is cooked on a normal stove. (3)

The first documented use of the term “microwave” was in 1931. It was used in a Telegraph and Telephone Journal, which said “When trials with wavelengths as low as 18 cm were made known, there was undisguised surprise that the problem of the micro-wave had been solved so soon.” (3)

5 thoughts on “Easy Mac, Spaghettios, and left overs…What would we do without microwave ovens?”

I would like to add a few more interesting facts about microwaves ovens-

-As the author mentioned, the microwaves can be reflected and absorbed by the metal interior of the microwave oven. But if you take a closer look at the door of your microwave oven, you will find that there is a metal web, instead of a metal sheet, on the door. The fact is that, a metal web is sufficient to keep the microwaves from escaping. This is configuration is essentially a Faraday Cage (details here http://en.wikipedia.org/wiki/Faraday_cage). In simple words, a Faraday Cage locks electromagnetic fields (which are created by the microwaves) within it so that it keeps people away from electric shocks. You can find the application of Faraday Cage in a lot of places, for example, the metal fence surrounding huge industrial electricity transformers.

-The microwave can excite polarized molecules, and it can also excite the free electrons in a piece of metal, which can create electric current in the metal and heat will accumulate – so next time, remove the tinfoil wrapper on your food before you put it in the microwave oven; you do not want your food and your microwave oven burn.

-The reflected microwaves within the microwave oven form a state called standing wave – the best analogy would be the vibrating string on a violin. In the parts where the microwave “vibrates” more radically, there are more energy. Hence, there will be “sweat spots” in your microwave oven that heat your food faster than other parts. That is why sometimes part of your food is already very hot but some other parts are not. Therefore, some of the microwave oven would have a plate, on which you put your food, that will rotate when you cook your food so that you do not have to stop the oven and move your food.

I really enjoyed your post! After reading a bit about how the microwave works, I was curious about some of the phenomena we try to avoid in the microwave.

The first I looked into was why you cannot microwave tin foil. When a microwave is typically used, the electromagnetic waves enter the food. The energy from the waves is converted into molecular vibrations and heat that cooks microwaveable foods. In the instance of tin foil, the metal does not take in this energy and vibrate like microwaveable foods do. This causes energy to build up that has to find a way to be released. Often this occurs as a spark into the open air in the microwave and sometimes becomes a small fire if left unchecked. Bill Nye weighs in on this subject at http://www.ccmr.cornell.edu/education/ask/index.html?quid=582.

They swell up, and if a package is put in the microwave, it could cause quite a mess! In this case, rather than failing to heat up, as the metal does, the Peep takes in the heat very well. A large portion of a Peep is water, that takes in a lot of energy from the microwave. This causes the Peep to heat quickly. The air contained within the Peep expands when heated, causing the Peep to expand at a rapid rate. For more information, check out the following sites.

Hey Sarah,
I thought that this was really interesting, and it is definitely applicable to my life considering how often I use my microwave! After reading your post and how microwaves heat things up by excited polarized molecules, I started thinking about why some containers/bowls/dishes aren’t recommended for use in the microwave. Personally, I know that we have some ceramic bowls and mugs at my house that get really hot when they are placed in the microwave. From what I found, there are some glazes that contain metals, and it is a bad idea to microwave these for the same reason that tin foil shouldn’t be microwaved (sparks, fire, etc.). And I also found that some bowls get hot in the microwave because the glazes or coatings on those actually reflect a decent portion of the microwaves. This causes the bowl to get hotter faster than what is actually in the bowl.http://www.last-word.com/content_handling/show_tree/tree_id/2663.html

Sarah,
I really like your post. It was a little physics-y for me at times, but I got the main points of it. After reading this I then decided to check out the differences between microwaves and ovens. I initially thought that they were all kind of the same thing, but upon doing my research I found that is not the case at all. As you said, microwaves use microwave radiation which is absorbed by the molecules of water and other substances within the food causing them to “shake” and heat up the food. On the other hand, ovens use coiled wires or open flames to heat the air surrounding them and inevitably the food (outside to in).
These differences mean that microwaves tend to heat foods more quickly and are why they are such a convenience.

This post reminded me of a myth busters episode that I saw over Thanksgiving break. They knew how microwaves worked, but they wanted to test whether other transmitters of electromagnetic radiation could cook a turkey just as well as a microwave could (although I don’t recommend a microwave for Thanksgiving dinner). They tried radio antenna, radar, etc, but they could not get the turkey to cook.

The reason is that the other forms that they had were not concentrated or energetic enough. You mentioned the equation E=hc/λ. Radio waves have a longer wavelength so they are less energetic, and the energy is not concentrated in a box like a microwave but instead broadcast across the country.